When using refrigerant in different contexts, it's often desirable to transfer the heat from a fluid to another without physically mixing the fluids. This may be because one is dirty or that it is simply different types of cold media. Counter-flow heat exchangers (CFHEs) are among the most energy efficient.
This type of flow arrangement allows the largest change in temperature of both fluids and is therefore most efficient (where efficiency is the amount of actual heat transferred compared with the theoretical maximum amount of heat that can be transferred).
Referring to ex FIG. 27, one can see that cold fluid automatically gets exposed to hotter sections the hotter the fluid gets, and it is possible for the heated cold fluid to have an output temperature (TcOut) to be close to the Hot fluid input temperature (ThIn).
Attempting to accomplish the same result with two gases, alternatively warming up a gas using a liquid with a CFHE, raises a number of problems.
Referring to FIG. 27, given two gases pumped counter flowingly in a heat exchanger, in the flow moving from cold to warm, the gas at the front would be hottest and thus get higher pressure. Due to the pressure difference in the cold and hot gas, some of the hot gas will tend to move/expand backwards against the intended flow direction (se arrows pointing right in flow B), resulting in some mixing of the cold gas and hot gas and counteracting the flow. Furthermore, the expansion will cool down the gas that is supposed to be heated. To summarize you tend to get an output temperature TcOut lower than, if it had been liquid, due to expansion and mixture of cold and hot gas in the heated flow (B), also often with lower output density (DcOut) which often can be un-preferred.
The interest in developing an apparatus that performs something functionally similar to a counterflow heat exchanger that works well on gas should be of interest. To show that this is a real and hard-to-solve problem, and not “obvious to a person skilled in the art”, refer to Wikipedia, where at some point it was said that “counterflow heat exchangers do not work well when heating gas” In addition, for the purpose of explaining the difficulty of the problem, a number of solutions to the problem will be described which at first glance may seem to work well, describing the disadvantages of these solutions. Wikipedia is not a scientific journal or the like, but at least it gives an indication that an effective equivalent of a counterflow heat changer for heating gas is difficult to create.
The following problems also occurs in when trying to perform the same task with a small amount of liquid flowing counterflowly against a gas being heated up (referring to FIG. 27, this would mean that flow A is liquid).
1. Referring to FIG. 32, if you were to use a fan (F) to press the cold gas towards a high pressure flow, (assuming the density Dcin=Dcout) work has to be added by the fan since the pressure (Pcout) will increase with constant density and increased temperature
2. Referring to FIG. 27, if you, when using a fan, allow the heated gas to expand to such an extent that the heated gas has the same pressure as the cold gas (Dcin>Dcout), you might not have to add, the above described, extra work, and the gas might not counteract the desired flow, but we are then talking about isobar heating of the gas, which requires more energy for the same degree of heating compared to isochor heating. Furthermore, the heated gas will have lower density than the cold gas, which in some cases aren't desirable.
3. Separating the different parts, each with a different temperature with check valves, only allowing movements from cold to hot, the gases will not be mixed. However, the warmer parts will have high pressure and, as in point 1, work must be added to press in the cold gas towards the warmer gas. Furthermore, check valves usually have a certain cracking pressure, which means you have to apply force to overcome that cracking pressure.
4. Another problem which would interfere with this procedure is that gas has low mass compared with the heat exchanger. To heat up the gas one must, at least, warm up the wall between the gas and liquid to be able to lead the heat to the gas. If a certain position of a walls are cooled/heated interchangedly, this energy has to be added, and the mass of the wall is probably large compared to the gas.
In order to describe this problem (4), a few less appropriate solutions to problem 1 and 3 are described,
Assume you try to address the problem colder gas mixing with hotter while heating by moving a whole volume of gas. The whole volume having substantially the same temperature at given point in time. Referring to FIG. 33, assume that the cup (C) in the FIG. 33 contains a gas to be heated and that gas volume is sealingly enclosed on the board (T). The board (T) comprises temperatures gradationally increasing from position A (coldest) to B (hottest). By sliding the cup along the board from A to B, the gas will be exposed to incrementally higher temperatures, as in a counterflow heat exchanger, still without having to apply any work on the gas to move it towards hotter sections, except for friction. The drawback is off course that you heat up the cup at the same time, and the mass of the cup is likely bigger than the gas.
In another solution the gas is stationary, and a fluid of varying temperature is flushed through the container, taking the heat from one gas and transferring it to another gas. This solves the problem of the gas counteracting the flow since the gas is stationary and instead liquid is flowing through the container. This raises a number of other issues instead due to the fact that you have to warm up the container while warming up the gas. The disadvantages of this solution are described below:
Referring to FIG. 28: A half bad solution for an optimal heat exchanger is a long flow of liquid refrigerant with a maximum temperature of =<Tmaxgas−Tt (the minimum temperature difference required for heat transfer) continuously declining to a minimum gas temperature, Tmingas.
With the temperature falling decrementally, depending on the position in the liquid flow will automatically adjust to the optimum temperature difference Td for current flow rate.
If hot gas container GC1 is to transfer heat to a cold gas container GC2 this can be accomplished by this long fluid passing through a container with incrementally decreasing (or increasing depending on which direction you flow the liquid) temperatures. Assume the long fluid have temperatures varying from close to Tcg1 (hot) decreasingly down to Tcg2 (cold).
Let's assume this fluid is flushed through CG1 exposing it to colder and colder temperatures as CG1 loses its heat, as it would if it was cooled in a Counterflow Heat exchanger. In every position of the fluid, the fluid will increase slightly in temperature, unless the temperature difference is less than Tt. Assume the max temperature after the flushing to be Tcg1max−Td, meaning it was less or equal to this when it started.
This extra heat in the fluids different positions is then to be transferred to the cold gas container GC2. This gas container (‘the containers are also assumed to be a heat exchangers) is the flushed with the same fluid backwards. Meaning it gets exposed to fluid of increasing temperatures in small steps, as if the gas was heated in a CFHE, without the turbulence/mixing/added work-problems described above. Assume Tt=0.5, meaning that the temperature of the gas in GC2 (with the above assumption) will maximally be Tcg1max−(2*Tt). The problem is that the container GC2, or at least part of the container will also be heated. To heat up the gas in GC2 to be Tgc1max, you also have to heat up the container GC2 or at least part of the container (minimum the wall between gas and liquid).
So to heat up GC2 you need to add the energy (Mass of GC2+Mass of Gas)*specific heat capacity*1. Had we “only” cooled down and warmed up the gas, i.e. completely isolated the heat exchanger from heat change, the energy needed would “only” have to be: (Mass of Gas)*specific heat capacity*1. But how can that be done? At least the heat-conducting wall between gas and liquid in a heat exchanger must be warmed up, otherwise it does not transfer the heat from the fluid to the gas.
If instead each position in a heat exchanger have a target temperature, which it reaches at equilibrium; this position loses energy to the gas that is heated, the lost energy in the heat exchanger is then added by the warmer refrigerant, i.e. only the same energy which is delivered to the gas (heated) needs to be added to the container and no more. That means that because the walls with mainly fixed temperature are still, gas need to be moved toward the warmer positions instead, with the problems that this entails.
The problem remains on how to move the gas towards positions exposing it to higher temperatures, without gas turbulence and mixing of cold and hot gas, without having to add force to pump it there, without having to heat the elements responsible for the movements.
The present invention will solve these problems and also display an appropriate method to transfer the heated gas to another device or a tank.